Wide spectrum optical sensor

ABSTRACT

An optical sensor including a semiconductor substrate; a first light absorption region formed in the semiconductor substrate, the first light absorption region configured to absorb photons at a first wavelength range and to generate photo-carriers from the absorbed photons; a second light absorption region formed on the first light absorption region, the second light absorption region configured to absorb photons at a second wavelength range and to generate photo-carriers from the absorbed photons; and a sensor control signal coupled to the second light absorption region, the sensor control signal configured to provide at least a first control level and a second control level.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of and claims the benefit toU.S. patent application Ser. No. 15/248,618, filed Aug. 26, 2016, whichclaims the benefit of U.S. Provisional Patent Application No.62/210,946, filed Aug. 27, 2015, U.S. Provisional Patent Application No.62/210,991, filed Aug. 28, 2015, U.S. Provisional Patent Application No.62/211,004, filed Aug. 28, 2015, U.S. Provisional Patent Application No.62/216,344, filed Sep. 9, 2015, U.S. Provisional Patent Application No.62/217,031, filed Sep. 11, 2015, U.S. Provisional Patent Application No.62/251,691, filed Nov. 6, 2015, and U.S. Provisional Patent ApplicationNo. 62/271,386, filed Dec. 28, 2015, which are incorporated by referenceherein.

BACKGROUND

This specification relates to detecting light using a semiconductorbased light absorption apparatus.

Light propagates in free space or an optical medium is coupled to asemiconductor based light absorption apparatus that converts an opticalsignal to an electrical signal for processing.

SUMMARY

A semiconductor based light absorption apparatus, such as a photodiode,may be used to detect optical signals and convert the optical signals toelectrical signals that may be further processed by another circuitry.Such light absorption optical sensor may be used in consumer electronicsproducts, image sensors, data communications, time-of-flight (TOF)applications, medical devices, security/surveillance, and many othersuitable applications. Conventionally, silicon is used as an imagesensor material, but silicon has a low optical absorption efficiency forwavelengths in the near-infrared (NIR) spectrum or longer. Othermaterials and/or material alloys such as germanium, germanium-silicon,or germanium-tin may be used as image sensor materials with innovativeoptical device structure design described in this specification.According to one innovative aspect of the subject matter described inthis specification, a photodiode is formed using materials such asgermanium or germanium-silicon to increase the speed and/or thesensitivity and/or the dynamic range and/or the operating wavelengthrange of the device. In one embodiment, photodiodes formed usinggermanium or germanium-silicon and photodiodes formed using silicon maybe integrated on a common substrate to yield a photodiode array having agreater operating wavelength range.

In general, one innovative aspect of the subject matter described inthis specification can be embodied in an optical that includes asemiconductor substrate; a first light absorption region formed in thesemiconductor substrate, the first light absorption region configured toabsorb photons at a first wavelength range and to generatephoto-carriers from the absorbed photons; a second light absorptionregion on the first light absorption region, the second light absorptionregion configured to absorb photons at a second wavelength range and togenerate photo-carriers from the absorbed photons; and a sensor controlsignal coupled to the second light absorption region, the sensor controlsignal configured to provide at least a first control level and a secondcontrol level. At the first control level, an energy band difference atan interface of the first light absorption region and the second lightabsorption region is above a threshold for blocking one specificpolarity of the photo-carriers generated by the second light absorptionregion from entering the first light absorption region. At the secondcontrol level, the energy band difference at the interface of the firstlight absorption region and the second light absorption region is belowthe threshold for blocking the one specific polarity of thephoto-carriers generated by the second light absorption region fromentering the first light absorption region.

This and other implementations can each optionally include one or moreof the following features. The first light absorption region may includean n-doped silicon region; and a p-doped silicon region on the n-dopedsilicon region. The second light absorption region may include anintrinsic region including germanium on the p-doped silicon region ofthe first light absorption region; and a p-doped region includinggermanium on the intrinsic region. The optical sensor may include ann-doped readout region coupled to a readout circuit; and a gate coupledto a gate control signal, the gate configured to control a carriertransit between the first light absorption region and the n-dopedreadout region. The second light absorption region may include a mesaincluding germanium. The second light absorption region may include afilm including germanium.

Another innovative aspect of the subject matter described in thisspecification can be embodied in an optical sensor that includes a firstdiode formed using a first material, the first diode comprising ann-doped region and a p-doped region; a NMOS transistor that includes asource region coupled to the n-doped region of the first diode; a gateregion coupled to a NMOS gate control signal; and a drain region; asecond diode formed using a second material, the second diode includingan n-doped region coupled to a first bias signal; and a p-doped region;and a PMOS transistor including a source region coupled to the p-dopedregion of the first diode and the p-doped region of the second diode; agate region coupled to a PMOS gate control signal; and a drain region.

This and other implementations can each optionally include one or moreof the following features. The drain region of the NMOS transistor maybe coupled to a first readout circuit. The drain region of the PMOStransistor may be coupled to a second bias source, such that (i) thefirst readout circuit collects, stores, and processes electronsgenerated by the first diode, (ii) the drain region of the PMOStransistor transfers holes generated by the first diode to the secondbias source, and (iii) the drain region of the PMOS transistor transfersholes generated by the second diode to the second bias source. The firstdiode may be configured to absorb light at visible wavelengths togenerate electrons and holes.

The drain region of the PMOS transistor may be coupled to a secondreadout circuit. The drain region of the NMOS transistor may be coupledto a third bias source, such that (i) the drain region of the NMOStransistor transfers electrons generated by the first diode to the thirdbias source, (ii) the second readout circuit collects, stores, andprocesses holes generated by the first diode, and (iii) the secondreadout circuit collects, stores, and processes holes generated by thesecond diode. The second diode may be configured to absorb light atnear-infrared or infrared wavelengths to generate electrons and holes.

The optical sensor may further include a substrate, where the firstdiode, the NMOS transistor, and the PMOS transistor are formed in thesubstrate. The second diode may further include an intrinsic region,where the p-doped region of the second diode is on the p-doped region ofthe first diode, where the intrinsic region of the second diode is onthe p-doped region of the second diode, and where the n-doped region ofthe second diode is on the intrinsic region of the second diode. Thefirst diode may be a diode including silicon and the second diode may bea diode including germanium.

Another innovative aspect of the subject matter described in thisspecification can be embodied in an optical sensor that includes asemiconductor substrate; a first light absorption region formed in thesemiconductor substrate, the first light absorption region configured toabsorb photons at a first wavelength range and to generatephoto-carriers from the absorbed photons, the first light absorptionregion including: a first carrier-collection region configured tocollect electrons; and a second carrier-collection region configured tocollect holes; a second light absorption region on a portion of thefirst light absorption region, the second light absorption regionconfigured to absorb photons at a second wavelength range and togenerate photo-carriers from the absorbed photons; a first readoutregion coupled to a first readout circuitry, the first readout regionconfigured to provide the electrons collected by the firstcarrier-collection region to the first readout circuitry, where theelectrons collected by the first carrier-collection region are providedby the first light absorption region; a first gate coupled to a firstcontrol signal that controls a carrier transport between the firstcarrier-collection region and the first readout region; a second readoutregion coupled to a second readout circuitry, the second readout regionconfigured to provide the holes collected by the secondcarrier-collection region to the second readout circuitry, where theholes collected by the second carrier-collection region are provided bythe second light absorption region; and a second gate coupled to asecond control signal that controls a carrier transport between thesecond carrier-collection region and the second readout region.

This and other implementations can each optionally include one or moreof the following features. The second light absorption region mayinclude a p-doped region including germanium on the first lightabsorption region; an intrinsic region including germanium on thep-doped region; and an n-doped region including germanium on theintrinsic region. The p-doped region may have a first strain and a firstarea, and the intrinsic region may have a second strain that is lowerthan the first strain, and the intrinsic region may have a second areathat is larger than the first area.

The second light absorption region may be on a portion of the secondcarrier-collection region but not on the first carrier-collectionregion. The first light absorption region and the second lightabsorption region may be configured to receive light at differentlocations. The first light absorption region and the second lightabsorption region may be coupled by one or more interconnects formed bybonding two donor wafers.

The optical sensor may include a third readout region coupled to a thirdreadout circuitry, the third readout region configured to provide theholes collected by the second carrier-collection region to the thirdreadout circuitry; and a third gate coupled to a third control signalthat controls a carrier transport between the second carrier-collectionregion and the third readout region. The optical sensor may include afourth readout region coupled to a fourth readout circuitry, the fourthreadout region configured to provide the holes collected by the secondcarrier-collection region to the fourth readout circuitry; and a fourthgate coupled to a fourth control signal that controls a carriertransport between the second carrier-collection region and the fourthreadout region.

Advantageous implementations may include one or more of the followingfeatures. Germanium is an efficient absorption material fornear-infrared wavelengths, which reduces the problem of slowphoto-carriers generated at a greater substrate depth when aninefficient absorption material, e.g., silicon, is used. An increaseddevice bandwidth allows the use of a higher modulation frequency in anoptical sensing system, giving advantages such as a greater depthresolution and a higher frame rate with innovative device design. Analloy germanium-silicon material as the optical absorption layer withinnovative device design provides higher optical absorption efficiencyover conventional Si material, which may provide a more sensitive sensorin the visible and near-infrared spectrums, may reduce crosstalk betweenneighboring pixels, and may allow for a reduction of pixel sizes. Apunch-through (or reach-through) sensor design may enable a single-pixelphotodiode or an array-pixel photodiodes to detect light in both thevisible and near-infrared spectrums. A hybrid sensor design may supporttime-of-flight (TOF), near-infrared, and visible image sensing withinthe same sensing array. An increased device bandwidth allows the use ofa higher modulation frequency in a time-of-flight system, giving agreater depth resolution. The punch-through sensor design and the hybridsensor design may be controlled to prevent dark currents from thegermanium-silicon region to leak into the silicon region and so avoidsperformance degradation. Having one sensor with two modes (e.g., VIS andNIR modes) instead of needing two separate sensors may reducemanufacturing cost, may increase the number of usable pixels in a givenarea, and may reduce packaging complexity.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other potentialfeatures and advantages will become apparent from the description, thedrawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example of a photodiode array.

FIGS. 2-4 are examples of a photodiode having a punch-through structurefor detecting visible and/or NIR light.

FIG. 5 is an example of a photodiode for detecting visible and/or NIRlight.

FIGS. 6A-6C are examples of a circuit schematic that represents thephotodiode described in reference to FIG. 5.

FIGS. 7-10 are examples of a multi-gate photodiode for detecting visibleand/or NIR light.

FIG. 11 is an example of an integrated photodiode for detecting visibleand/or NIR light.

FIG. 12 is an example of an integrated photodiode for detecting visibleand/or NIR light.

FIGS. 13-14 are examples of a multi-gate photodiode for detectingvisible and/or NIR light.

FIG. 15 is an example of a multi-gate photodiode.

FIGS. 16A-16D illustrate an example of a design with exemplaryfabrication of a photodiode array.

Like reference numbers and designations in the various drawings indicatelike elements. It is also to be understood that the various exemplaryembodiments shown in the figures are merely illustrative representationsand are not necessarily drawn to scale.

DETAILED DESCRIPTION

Semiconductor based optical sensors, such as photodiodes may be used todetect optical signals and convert the optical signals to electricalsignals that may be further processed by another circuitry. In general,a material absorbs light at various wavelengths to generate freecarriers depending on an energy bandgap associated with the material.For example, at room temperature, silicon (Si) may have an energybandgap of 1.12 eV. As another example, at room temperature, germanium(Ge) may have an energy bandgap of 0.66 eV, and a germanium-siliconalloy (Ge_(x)Si_(1-x)) may have an energy bandgap between 0.66 eV and1.12 eV depending on the composition. As another example, at roomtemperature, tin (Sn) may have an energy bandgap of about 0 eV, and agermanium-tin alloy (Ge_(x)Sn_(1-x)) may have an energy bandgap between0 eV and 0.66 eV depending on the composition. In general, a materialhaving a lower energy bandgap has a higher absorption coefficient atwider wavelength ranges. If the absorption coefficient of a material istoo low, the optical signal cannot be converted to an electrical signalefficiently. However, if the absorption coefficient of a material is toohigh, free carriers will be generated near the surface of the materialwith reduced efficiency. Silicon is not an efficient sensor material forNIR wavelengths due to its large bandgap. On the other hand, germaniumor tin has an absorption coefficient that may be too high for shorterwavelengths (e.g., blue), where free carriers may recombine at thesurface. An optical sensor that integrates silicon and a differentmaterial, such as germanium or germanium-silicon, germanium-tin, on acommon substrate, where the optical sensor uses silicon to detectvisible light and uses the different material to detect NIR light, wouldenable the optical sensor to have a wide detection spectrum. In thisapplication, the term “photodiode” may be used interchangeably as theterm “optical sensor”. In this application, the term “germanium-silicon(GeSi)”, “silicon-germanium (SiGe)” may be used interchangeably, andboth include all suitable SiGe composition combinations from 100%germanium (Ge) to more than 90% silicon (Si). In this application, theterm “germanium-tin (GeSn)” includes all suitable GeSn compositioncombinations from 100% germanium (Ge) to more than 90% tin (Sn). In thisapplication, the GeSi layer may be formed using blanket epitaxy,selective epitaxy, or other applicable techniques. Furthermore, the GeSilayer may be formed on a planar surface, a mesa surface, or atrench/hole region at least partially surround be insulator (ex: oxide,nitrite), or semiconductor (ex: Si, Ge), or their combinations. Alsonote that in this application, lightly doped region may have dopinglevel from 10¹⁵ to 10¹⁹ cm⁻³. Furthermore, a strained super latticestructure including multiple layers such as alternating GeSi layer withdifferent compositions may be used for the absorption or forming aquantum well structure.

FIG. 1 is an example a photodiode array 100 where photodiodes are formedby integrating silicon with germanium-silicon. An optical image sensorarray is an example of a photodiode array. The photodiode array 100includes a substrate 102, an integrated circuit layer 104, aninterconnect layer 106, a sensor layer 108, a filter layer 110, and alens layer 112. In general, light of a single wavelength or multiplewavelengths enters the lens layer 112, where the light may be focused,collimated, expanded, or processed according to the lens design. Theshape of the lens could be concave, convex, or planar with surfacestructure such as Fresnel lens, or other shapes, and its shape shouldnot be limited by the exemplary drawings here.

The light then enters the filter layer 110, where the filter layer 110may be configured to pass light having one or more specific wavelengthranges. A filter may be formed by depositing layers of dielectricmaterials, such that light having a wavelength within a specificwavelength range would pass through the filter and light having awavelength outside the specific wavelength range would be reflected bythe filter. For example, a filter of the filter layer 110 may beconfigured to pass light at a blue wavelength range (e.g., 460 nm±40 nm)and a NIR wavelength range (e.g., 850 nm±40 nm, 940 nm±40 nm, or >1 μm),while another filter of the filter layer 110 may be configured to passlight at a green wavelength range (e.g., 540 nm±40 nm) and the NIRwavelength range. In some implementation, one or more filters of thefilter layer 110 may pass wavelengths in a tunable fashion by includinga capacitive microelectromechanical systems (MEMS) structure. Thephotodiodes in the sensor layer 108 converts the incident light intofree carriers. The integrated circuit layer 104 collects the freecarriers through the interconnect layer 106 and processes the freecarriers according to the specific application.

In general, the substrate 102 may be a silicon substrate, asilicon-on-insulator (SOI) substrate, or any other suitable carriersubstrate materials. The integrated circuits of the integrated circuitlayer 104 and the interconnects of the interconnect layer 106 may befabricated using CMOS processing techniques. For example, theinterconnects may be formed by dry-etching a contact region through adielectric layer in the form of holes or trenches and filling thecontact region by copper using electroplating process.

The sensor layer 108 includes multiple photodiodes for detecting visibleand/or NIR light. Each photodiode may be isolated by insulating sidewallspacers, trenches, or other suitable isolation structures. In someimplementations, the wavelength range that a photodiode is configured todetect may be controlled by an optical filter in the filter layer 110.For example, a photodiode may be configured to receive a red wavelengthrange (e.g., 620 nm±40 nm) and a NIR wavelength range, where the centerwavelengths and the limits of the wavelength ranges are controlled bythe characteristics of the filter above the photodiode. In someimplementations, the wavelength range that a photodiode is configured todetect may be controlled by a material composition of the photodiode.For example, an increase in germanium composition in a germanium-siliconalloy may increase the sensitivity of the photodiode at longerwavelengths. In some implementations, the wavelength range that aphotodiode is configured to detect may be controlled by a combination ofthe optical filter and the material composition of the photodiode.

As described in more details in reference to FIGS. 2-4, in someimplementations, the photodiodes in the photodiode array 100 may becontrolled to transfer photo-carriers generated by either the visible orthe NIR light to a readout circuit for further processing. For example,the photodiodes in the photodiode array 100 may be controlled totransfer photo-carriers generated by the visible light to the readoutcircuit for further processing during daytime, and the photodiodes maybe controlled to transfer photo-carriers generated by the NIR light tothe readout circuit for further processing during night time fornight-vision applications. As another example, the optical sensor can becontrolled to collect both the visible light and the NIR lightsimultaneously or in an alternating sequence.

As described in more details in reference to FIGS. 5-10, in someimplementations, the photodiodes in the photodiode array 100 may becontrolled to transfer photo-carriers generated by either the visible orthe NIR light to different readout circuits for further processing. Forexample, the photodiodes in the photodiode array 100 may be controlledto transfer free-electrons generated by the visible light to a firstreadout circuit and to transfer free-holes generated by the NIR light toa second readout circuit for further processing. As another example, theoptical sensor can be controlled to collect both the visible light andthe NIR light simultaneously or in an alternating sequence.

As described in more details in reference to FIGS. 13-14, in someimplementations, different photodiodes in the photodiode array 100 maybe controlled to detect the visible and/or the NIR light. For example,one pixel of the photodiodes in the photodiode array 100 may becontrolled to transfer free-electrons generated by the visible light toa first readout circuit, while another pixel of the photodiode in thephotodiode array 100 may be controlled to transfer free-holes generatedby the NIR light to a second readout circuit for further processing. Insome implementations, both pixels may share part of the structures suchas the doping region.

FIG. 2 illustrates an example photodiode 200 having a punch-throughstructure for detecting visible and near-infrared optical signals. Ingeneral, the example photodiodes 200 includes a first absorption regionfor converting a visible optical signal to photo-carriers (i.e.,electron-hole pairs) and a second absorption region for converting a NIRoptical signal to photo-carriers. A sensor control signal 226 maycontrol the transfer of either photo-carriers generated by the visibleoptical signal or photo-carriers generated by the NIR optical signal tothe readout circuit 224. The photodiode 200 may be one of thephotodiodes in the sensor layer 108 as described in reference to FIG. 1,for example.

The photodiode 200 includes a p-Si substrate 202, a p+ Si region 204, ann-Si region 206, a p-Si region 208, an intrinsic GeSi region 210, a p+GeSi region 212, a gate 214, an n+ Si region 216, a gate control signal222 coupled to the gate 214, a sensor control signal 226 coupled to thep+ GeSi region 212, and a readout circuit 224 coupled to the n+ Siregion 216. In some implementations, the first absorption region forconverting a visible optical signal to photo-carriers may include the p+Si region 204, the n-Si region 206, and the p-Si region 208. The p+ Siregion 204 may have a p+ doping, where the activated dopantconcentration may be as high as a fabrication process may achieve, e.g.,about 5×10²⁰ cm⁻³ with boron. The n-Si region 206 may be lightly dopedwith an n-dopant, e.g., phosphorus. The p-Si region 208 may be lightlydoped with a p-dopant, e.g., boron. In some implementations, the secondabsorption region for converting a NIR optical signal to photo-carriersmay include the intrinsic GeSi region 210 and the p+ GeSi region 212.The p+ GeSi region 212 may have a p+ doping, where the dopantconcentration may be as high as a fabrication process may achieve, e.g.,about 5×10²⁰ cm⁻³ when the intrinsic GeSi region 210 is germanium anddoped with boron. Here, the intrinsic GeSi region 210 is agermanium-silicon film. The p-Si substrate 202 may be lightly doped witha p-dopant, e.g., boron.

In general, the first absorption region of the photodiode 200 receivesthe optical signal 220. The optical signal 220 may be a visible opticalsignal or an NIR optical signal. In some implementations, the opticalsignal 220 may be filtered by a wavelength filter not shown in thisfigure, such as a filter in the filter layer 110 as described inreference to FIG. 1. In some implementations, a beam profile of theoptical signal 220 may be shaped by a lens not shown in this figure,such as a lens in the lens layer 112 as described in reference toFIG. 1. In the case where the optical signal 220 is a visible opticalsignal, the first absorption region may absorb the visible opticalsignal and generate photo-carriers. In the case where the optical signal220 is a NIR optical signal, the first absorption region may betransparent or nearly transparent to the NIR optical signal, and thesecond absorption region may absorb the NIR optical signal and generatephoto-carriers. The sensor control signal 226 may control whether thephotodiode 200 operates in a visible light mode or in a NIR light modewhen a tunable wavelength filter is applied.

In the visible light mode, the optical signal 220 is a visible opticalsignal, where the first absorption region absorbs the visible opticalsignal and generates photo-carriers. A built-in potential between the p+Si region 204 and the n-Si region 206 creates an electric field betweenthe two regions, where free electrons generated from the n-Si region 206are drifted/diffused to a region below the p+ Si region 204 by theelectric field. The p-Si region 208 is configured to repel thefree-electrons in the intrinsic GeSi region 210 from entering the firstabsorption region, such that the dark current of the photodiode 200 maybe reduced. The sensor control signal 226 is configured to be able toapply a first sensor control signal level to further preventfree-electrons in the intrinsic GeSi region 210 from entering the firstabsorption region, such that the dark current of the photodiode 200 maybe further reduced. For example, the p+ GeSi region 212 may be coupledto a voltage source, where the sensor control signal 226 may be atime-varying voltage signal from the voltage source. The first sensorcontrol signal level may be 0V, such that an energy band differencebetween the p-Si region 208 and the intrinsic GeSi region 210 furtherprevents free-electrons in the intrinsic GeSi region 210 from enteringthe p-Si region 208. In some implementations, an intrinsic Si layer orunintentionally doped Si layer may be inserted in between the n-Si layer206 and p-Si layer 208 to increase the optical absorption of the firstabsorption region.

The gate 214 may be coupled to the gate control signal 222. For example,the gate 214 may be coupled to a voltage source, where the gate controlsignal 222 may be a time-varying voltage signal from the voltage source.The gate control signal 222 controls a flow of free electrons from theregion below the p+ Si region 204 to the n+ Si region 216. For example,if a voltage of the gate control signal 222 exceeds a threshold voltage,free electrons accumulated in the region below the p+ Si region 204 willdrift or diffuse to the n+ Si region 216.

The n+ Si region 216 may be coupled to the readout circuit 224. Thereadout circuit 224 may be in a three-transistor configurationconsisting of a reset gate, a source-follower, and a selection gate, orany suitable circuitry for processing free carriers. In someimplementations, the readout circuit 224 may be fabricated on asubstrate that is common to the photodiode 200. In some otherimplementations, the readout circuit 224 may be fabricated on anothersubstrate and co-packaged with the photodiode 200 via die/wafer bondingor stacking. For example, the readout circuit 224 may be fabricated onthe integrated circuit layer 104 as described in reference to FIG. 1.

In the NIR light mode, the optical signal 220 is a NIR optical signal,where the optical signal 220 passes through the first absorption regionbecause of the low NIR absorption coefficient of silicon, and the secondabsorption region then absorbs the NIR optical signal to generatephoto-carriers. In general, the intrinsic GeSi region 210 receives anoptical signal 220 and converts the optical signal 220 into electricalsignals. In some implementations, a thickness of the intrinsic GeSiregion 210 may be between 0.05 μm to 2 μm. In some implementations, theintrinsic GeSi region 210 may include a p+ GeSi region 212. The p+ GeSiregion 212 may repel the photo-electrons generated from the intrinsicGeSi region 210 to avoid surface recombination and thereby may increasethe carrier collection efficiency.

In the NIR light mode, the sensor control signal 226 is configured toapply a second sensor control signal level to allow free-electrons inthe intrinsic GeSi region 210 to enter the first absorption region. Forexample, the second sensor control signal level may be −1.2V, such thatthe p-Si region 208 is depleted, and the free-electrons in the intrinsicGeSi region 210 may enter the n-Si region 206 and drift or diffuse tobelow the p+ Si region 204. The free-electrons accumulated in the regionbelow the p+ Si region 204 may be transferred to the readout circuit 224by controlling the gate control signal 222 in similar manners asdescribed earlier in reference to the visible mode.

Although not shown in FIG. 2, in some other implementations, the firstabsorption region and the second absorption region of the photodiode 200may alternatively be designed into opposite polarity to collect holes.In this case, the p-Si substrate 202 would be replaced by an n-Sisubstrate, the p+ Si region 204 would be replaced by an n+ Si region,the n-Si region 206 would be replaced by a p-Si region, the p-Si region208 would be replaced by an n-Si region, the p-Si substrate 202 would bereplaced by an n-Si substrate, the n+ Si region 216 would be replaced bya p+ Si region, and the p+ GeSi region 212 would be replaced by an n+GeSi region.

FIG. 3 illustrates an example photodiode 300 having a punch-throughstructure for detecting visible and near-infrared optical signals. Ingeneral, the example photodiodes 300 includes a first absorption regionfor converting a visible optical signal to photo-carriers and a secondabsorption region for converting a NIR optical signal to photo-carriers.A sensor control signal 326 may control the transfer of eitherphoto-carriers generated by the visible optical signal or photo-carriersgenerated by the NIR optical signal to the readout circuit 324. Thephotodiode 300 may be one of the photodiodes in the sensor layer 108 asdescribed in reference to FIG. 1, for example.

The photodiode 300 includes a p-Si substrate 302, a p-Si region 304, ann-Si region 306, an intrinsic GeSi region 310, a p+ GeSi region 312, agate 314, an n+ Si region 316, a gate control signal 322 coupled to thegate 314, a sensor control signal 326 coupled to the p+ GeSi region 312,and a readout circuit 324 coupled to the n+ Si region 316. In someimplementations, the first absorption region for converting a visibleoptical signal to photo-carriers may include the p-Si region 304 and then-Si region 306. The n-Si region 306 may be lightly doped with ann-dopant, e.g., phosphorus. The p-Si region 304 may be lightly dopedwith a p-dopant, e.g., boron. In some implementations, the secondabsorption region for converting a NIR optical signal to photo-carriersmay include the intrinsic GeSi region 310 and the p+ GeSi region 312.The p+ GeSi region 312 may have a p+ doping, where the dopantconcentration may be as high as a fabrication process may achieve, e.g.,about 5×10²⁰ cm⁻³ when the intrinsic GeSi region 310 is germanium anddoped with boron. Here, the intrinsic GeSi region 310 is agermanium-silicon mesa. The p-Si substrate 302 may be lightly doped witha p-dopant, e.g., boron.

In general, the first absorption region of the photodiode 300 receivesthe optical signal 320. The optical signal 320 may be a visible opticalsignal or an NIR optical signal similar to the optical signal 220 asdescribed in reference to FIG. 2. In the case where the optical signal320 is a visible optical signal, the first absorption region may absorbthe visible optical signal and generate photo-carriers. In the casewhere the optical signal 320 is a NIR optical signal, the firstabsorption region may be transparent or nearly transparent to the NIRoptical signal, and the second absorption region may absorb the NIRoptical signal and generate photo-carriers. The sensor control signal326 may control whether the photodiode 300 operates in a visible lightmode or in a NIR light mode when a tunable wavelength filter is applied.

In the visible light mode, the optical signal 320 is a visible opticalsignal, where the first absorption region absorbs the visible opticalsignal and generates photo-carriers. A built-in potential between thep-Si region 304 and the n-Si region 306 creates an electric fieldbetween the two regions, where free electrons generated from the n-Siregion 306 are drifted/diffused to a region below the p-Si region 304 bythe electric field. The p-Si region 304 is configured to repel thefree-electrons in the intrinsic GeSi region 310 from entering the firstabsorption region, such that the dark current of the photodiode 300 maybe reduced. The sensor control signal 326 is configured to be able toapply a first sensor control signal level to further preventfree-electrons in the intrinsic GeSi region 310 from entering the firstabsorption region, such that the dark current of the photodiode 300 maybe further reduced. For example, the first sensor control signal levelmay be 0V, such that an energy band difference between the p-Si region304 and the intrinsic GeSi region 310 further prevents free-electrons inthe intrinsic GeSi region 310 from entering the p-Si region 304. In someimplementations, an i-Si or unintentionally doped Si layer may beinserted between the n-Si layer 304 and p-Si layer 306 to increase theoptical absorption of the first absorption region.

The gate 314 may be coupled to the gate control signal 322. The gatecontrol signal 322 controls a flow of free electrons from the regionbelow the p-Si region 304 to the n+ Si region 316. For example, if avoltage of the gate control signal 322 exceeds a threshold voltage, freeelectrons accumulated in the region below the p+ Si region 304 willdrift or diffuse to the n+ Si region 316. The n+ Si region 316 may becoupled to the readout circuit 324. The readout circuit 324 may besimilar to the readout circuit 224 as described in reference to FIG. 2.

In the NIR light mode, the optical signal 320 is a NIR optical signal,where the optical signal 320 passes through the first absorption regionbecause of the low NIR absorption coefficient of silicon, and the secondabsorption region then absorbs the NIR optical signal to generatephoto-carriers. In general, the intrinsic GeSi region 310 receives anoptical signal 320 and converts the optical signal 320 into electricalsignals. The p+ GeSi region 312 may repel the photo-electrons generatedfrom the intrinsic GeSi region 310 to avoid surface recombination andthereby may increase the carrier collection efficiency.

In the NIR light mode, the sensor control signal 326 is configured toapply a second sensor control signal level to allow free-electrons inthe intrinsic GeSi region 310 to enter the first absorption region. Forexample, the second sensor control signal level may be −1.2, such thatthe p-Si region 304 is depleted, and the free-electrons in the intrinsicGeSi region 310 may enter the n-Si region 306. The free-electronscollected in the n-Si region 306 may be transferred to the readoutcircuit 324 by controlling the gate control signal 322 in similarmanners as described earlier in reference to the visible mode.

Although not shown in FIG. 3, in some other implementations, the firstabsorption region and the second absorption region of the photodiode 300may alternatively be designed into opposite polarity to collect holes.In this case, the p-Si substrate 302 would be replaced by an n-Sisubstrate, the p-Si region 304 would be replaced by an n-Si region, then-Si region 306 would be replaced by a p-Si region, the p-Si substrate302 would be replaced by an n-Si substrate, the n+ Si region 316 wouldbe replaced by a p+ Si region, and the p+ GeSi region 312 would bereplaced by an n+ GeSi region.

FIG. 4 illustrates an example photodiode 400 having a punch-throughstructure for detecting visible and near-infrared optical signals. Ingeneral, the example photodiodes 400 includes a first absorption regionfor converting a visible optical signal to photo-carriers and a secondabsorption region for converting a NIR optical signal to photo-carriers.A sensor control signal 426 may control the transfer of eitherphoto-carriers generated by the visible optical signal or photo-carriersgenerated by the NIR optical signal to the readout circuit 424. Thephotodiode 400 may be one of the photodiodes in the sensor layer 108 asdescribed in reference to FIG. 1, for example.

The photodiode 400 includes a p-Si substrate 402, a p+ Si region 404, ann-Si region 406, a p-Si region 408, an intrinsic GeSi region 410, a p+GeSi region 412, a gate 414, an n+ Si region 416, a gate control signal422 coupled to the gate 414, a sensor control signal 426 coupled to thep+ GeSi region 412, and a readout circuit 424 coupled to the n+ Siregion 416. In some implementations, the first absorption region forconverting a visible optical signal to photo-carriers may include the p+Si region 404, the n-Si region 406, and the p-Si region 408. The p+ Siregion 404, the n-Si region 406, and the p-Si region 408 are similar tothe p+ Si region 204, the n-Si region 206, and the p-Si region 208 asdescribed in reference to FIG. 2. In some implementations, the secondabsorption region for converting a NIR optical signal to photo-carriersmay include the intrinsic GeSi region 410 and the p+ GeSi region 412.The p+ GeSi region 412 may have a p+ doping, where the dopantconcentration may be as high as a fabrication process may achieve, e.g.,about 5×10²⁰ cm⁻³ when the intrinsic GeSi region 410 is germanium anddoped with boron. Here, the intrinsic GeSi region 410 is a mesastructure that is surrounded by an insulator layer 430, and may befabricated by a selective Ge epitaxial growth.

In general, the first absorption region of the photodiode 400 receivesthe optical signal 420. The optical signal 420 may be a visible opticalsignal or an NIR optical signal similar to the optical signal 220 asdescribed in reference to FIG. 2. In the case where the optical signal420 is a visible optical signal, the first absorption region may absorbthe visible optical signal and generate photo-carriers. In the casewhere the optical signal 420 is a NIR optical signal, the firstabsorption region may be transparent or nearly transparent to the NIRoptical signal, and the second absorption region may absorb the NIRoptical signal and generate photo-carriers. The sensor control signal426 may control whether the photodiode 400 operates in a visible lightmode or in a NIR light mode similar to the visible light mode and theNIR light mode operations described in reference to FIG. 2.

Although not shown in FIG. 4, in some other implementations, the firstabsorption region and the second absorption region of the photodiode 400may alternatively be designed into opposite polarity to collect holes.In this case, the p-Si substrate 402 would be replaced by an n-Sisubstrate, the p+ Si region 404 would be replaced by an n+ Si region,the n-Si region 406 would be replaced by a p-Si region, the p-Si region408 would be replaced by an n-Si region, the p-Si substrate 402 would bereplaced by an n-Si substrate, the n+ Si region 416 would be replaced bya p+ Si region, and the p+ GeSi region 412 would be replaced by an n+GeSi region.

FIG. 5 illustrates an example photodiode 500 having a hybrid structurefor detecting visible and NIR optical signals. The example photodiodes500 includes a first diode for converting a visible optical signal tophoto-carriers, a second diode for converting a NIR optical signal tophoto-carriers, an NMOS transistor for transporting the photo-carriers,primarily electrons, generated by the first diode to a first readoutcircuit, and a PMOS transistor for transporting the photo-carriers,primarily holes, generated by the second diode to a second readoutcircuit. The photodiode 500 may be one of the photodiodes in the sensorlayer 108 as described in reference to FIG. 1, for example.

The first diode may include an n-Si region 504 and a p-Si region 506fabricated in a p-Si substrate 502 that is lightly doped with ap-dopant, e.g., boron. The n-Si region 504 may be lightly doped with ann-dopant, e.g., phosphorus. The p-Si region 506 may be lightly dopedwith a p-dopant, e.g., boron. The NMOS transistor may include a first n+Si region 514, a p-Si region 518, a second n+ Si region 516, and an NMOSgate 520. An NMOS gate control signal 522 may be coupled to the NMOSgate 520, and a first readout circuit 524 may be coupled to the secondn+ Si region 516.

The second diode may include a p-GeSi region 508, an intrinsic GeSiregion 510, and an n+ GeSi region 512. In some implementations, athickness of the intrinsic GeSi region 510 may be between 0.05 μm to 2μm. The n+ GeSi region 512 may have a n+ doping, where the dopantconcentration may be as high as a fabrication process may achieve, e.g.,about 5×10²⁰ cm⁻³ when the intrinsic GeSi region 510 is germanium anddoped with phosphorus. The p-GeSi region 508 may be lightly doped with ap-dopant, e.g., boron, when the intrinsic GeSi region 510 is germanium.The PMOS transistor may include a first p+ Si region 534, an n-Si region528, a second p+ Si region 536, and a PMOS gate 530. A PMOS gate controlsignal 538 may be coupled to the PMOS gate 530, and a second readoutcircuit 532 may be coupled to the second p+ Si region 536. Although notshown in FIG. 5, the n+ GeSi region 512 may have opposite polarity,namely become a p+ GeSi region to form a p-i-p vertical doping profilein the GeSi region.

FIGS. 6A-6C illustrate an example circuitry 600 for illustratingoperations of the photodiode 500 or other structures with similar devicedesign concepts. Referring to FIG. 6A, the circuitry 600 includes asilicon diode 606, a germanium-silicon diode 604, a PMOS transistor 602,and a NMOS transistor 608, which may correspond to the first diode, thesecond diode, the PMOS transistor, and the NMOS transistor as describedin reference to FIG. 5, respectively. The source of the NMOS transistor608 is coupled to the n-end of the silicon diode 606, the drain of theNMOS transistor 608 is coupled to a first readout circuit 624, and thegate of the NMOS transistor 608 is coupled to an NMOS gate controlsignal 622. The source of the PMOS transistor 602 is coupled to thep-end of the silicon diode 606 and the p-end of the germanium-silicondiode 604, the drain of the PMOS transistor 602 is coupled to a secondreadout circuit 632, and the gate of the PMOS transistor 602 is coupledto a PMOS gate control signal 638. The v-end of the germanium silicondiode 604 is coupled to a voltage source V_(DD). Although not shown inFIG. 6A, the first readout circuit 624 and the second readout circuit632 may each be in a three-transistor configuration consisting of areset gate, a source-follower, and a selection gate, or any suitablecircuitry for processing free carriers.

Referring to FIG. 6B, in the visible light mode, the drain of the PMOStransistor 602 is coupled to a voltage source V_(SS). This may beachieved by activating the reset gate of the second readout circuit 632to couple the drain of the PMOS transistor 602 to the voltage sourceV_(SS) of the second readout circuit 632. In the visible light mode, thesilicon diode 606 absorbs the incoming optical signal and generateselectron-hole pairs. For example, referring to FIG. 5, if the opticalsignal 540 is in the visible wavelength spectrum, the n-Si region 504would absorb the optical signal 540 to generate electron-hole pairs.Example values of V_(DD) and V_(SS) may be 1.2V and −1.2V.

Referring back to FIG. 6B, the free electrons generated by the silicondiode 606 may be transferred to the first readout circuit 624 by turningon the NMOS transistor 608 using the NMOS gate control signal 622. Forexample, referring to FIG. 5, by turning on the NMOS gate 520 using theNMOS gate control signal 522, the free electrons accumulated in the n-Siregion 504 may be transferred from the first n+ Si region 514 to thesecond n+ Si region 516, where the first readout circuit 524 maycollect, store, and process the electrons.

Referring back to FIG. 6B, the free holes generated by the silicon diode606 may be transferred to the voltage source V_(SS) by turning on thePMOS transistor 602 using the PMOS gate control signal 638. In general,it is desirable to minimize or eliminate the noise-current, such as thedark-current, from the germanium-silicon diode 604 in the visible lightmode because the dark-current of a germanium-silicon photodiode isgenerally much greater than the dark-current of a silicon photodiode.Since the n-end of the germanium-silicon diode 604 is coupled to avoltage source V_(DD), the free electrons generated by thegermanium-silicon diode 604 would be transferred to the voltage sourceV_(DD). Similarly, since the p-end of the germanium-silicon diode 604 iscoupled to a voltage source V_(SS) by letting the PMOS transistor on,the free holes generated by the germanium-silicon diode 604 would betransferred to the voltage source V_(SS). Consequently, thephoto-carriers generated by the germanium-silicon diode 604 would not betransferred to the first readout circuit 624, thereby improving theoverall performance of photodiode 600 in the visible light mode.

Referring to FIG. 6C, in the NIR light mode, the drain of the NMOStransistor 608 is coupled to a voltage source V_(DD). This may beachieved by activating the reset gate of the first readout circuit 624to couple the drain of the NMOS transistor 608 to the voltage sourceV_(DD) of the first readout circuit 624. In the NIR light mode, thegermanium-silicon diode 604 absorbs the incoming optical signal andgenerates electron-hole pairs. For example, referring to FIG. 5, if theoptical signal 540 is in the NIR wavelength spectrum, the n-Si region504 and the p-Si region 506 would be transparent to the optical signal540, and the intrinsic GeSi region 510 would absorb the optical signal540 to generate electron-hole pairs. Example values of V_(DD) may be1.2V.

Referring back to FIG. 6C, the free holes generated by thegermanium-silicon diode 604 may be transferred to the second readoutcircuit 632 by turning on the PMOS transistor 602 using the PMOS gatecontrol signal 638. For example, referring to FIG. 5, by turning on thePMOS gate 530 using the PMOS gate control signal 538, the free holesaccumulated in the p-Si region 506 may be transferred from the first p+Si region 534 to the second p+ Si region 536, where the second readoutcircuit 532 may collect, store, and process the holes.

Referring back to FIG. 6C, the free electrons generated by thegermanium-silicon diode 604 may be transferred to the voltage sourceV_(DD) that is coupled to the v-end of the germanium-silicon diode 604.Since the p-end of the silicon diode 606 is coupled to the PMOStransistor 602, the free holes generated by the silicon diode 606 wouldalso be transferred to the second readout circuit 632. This isacceptable because the dark current of a silicon diode is generally muchsmaller than the dark current of a germanium-silicon diode. Since then-end of the silicon diode 606 is coupled to a voltage source V_(DD) byletting the NMOS transistor on, the free electrons generated by thesilicon diode 608 would be transferred to the voltage source V_(DD).

Although not shown in FIGS. 5 and 6A, in some other implementations, thefirst diode (e.g., the silicon diode 606) and the second diode (e.g.,the germanium-silicon diode 604) may alternatively be designed intoopposite polarity to collect holes and electrons, respectively. In thiscase, the p-Si substrate 502 would be replaced by an n-Si substrate, thep-Si region 506 would be replaced by an n-Si region, the n-Si region 504would be replaced by a p-Si region, the p+ GeSi region 512 would bereplaced by an n+ GeSi region, the p-GeSi region 508 would be replacedby an n-GeSi region, the NMOS transistor would be replaced by a PMOStransistor, and the PMOS transistor would be replaced by an NMOStransistor. In some other implementations, the first diode and thesecond diode may be fabricated using other types of materials. Forexample, the second diode may be fabricated by a germanium-tin alloy todetect IR wavelength spectrum.

FIG. 7 illustrates an example photodiode 700 having a hybrid structurefor detecting visible and NIR optical signals. The example photodiodes700 includes a first absorption region for converting a visible opticalsignal to photo-carriers, and a second absorption region for convertinga NIR optical signal to photo-carriers. A first gate control signal 722controls the transfer of free-electrons generated by the firstabsorption region to a first readout circuit 724, and a second gatecontrol signal 738 controls the transfer of free-holes generated by thesecond absorption region to a second readout circuit 732. The photodiode700 may be one of the photodiodes in the sensor layer 108 as describedin reference to FIG. 1, for example.

In some implementations, the first absorption region may include an n-Siregion 704 and a p-Si region 706 fabricated in a p-Si substrate 702 thatmay be lightly doped with a p-dopant, e.g., boron. The n-Si region 704may be lightly doped with an n-dopant, e.g., phosphorus. The p-Si region706 may be lightly doped with a p-dopant, e.g., boron. The secondabsorption region may include a p-GeSi region 708, an intrinsic GeSiregion 710, and an n+ GeSi region 712. In some implementations, athickness of the intrinsic GeSi region 710 may be between 0.05 μm to 2μm. The n+ GeSi region 712 may have a n+ doping, where the dopantconcentration may be as high as a fabrication process may achieve, e.g.,about 5×10²⁰ cm⁻³ when the intrinsic GeSi region 710 is germanium anddoped with phosphorus. The p-GeSi region 708 may be lightly doped with ap-dopant, e.g., boron, when the intrinsic GeSi region 710 is germanium.

In general, the n-Si layer 704 receives an optical signal 740. If theoptical signal 740 is a visible optical signal, the n-Si region 704absorbs the optical signal 740 and converts the optical signal 740 intofree carriers. A built-in potential between the p-Si region 706 and then-Si region 704 creates an electric field between the two regions, wherefree electrons generated from the n-Si region 704 are drifted/diffusedtowards the region below the p-Si region 706 by the electric field.

A first gate 720 may be coupled to the first gate control signal 722.For example, the first gate 720 may be coupled to a voltage source,where the first gate control signal 722 may be a time-varying voltagesignal from the voltage source. The first gate control signal 722controls a flow of free electrons from the region below the p-Si region706 to the n+ Si region 716. For example, if a voltage of the controlsignal 722 exceeds a threshold voltage, free electrons accumulated inthe region below the p-Si region 706 will drift or diffuse to the n+ Siregion 716 for collection. The n+ Si region 716 may be coupled to thefirst readout circuit 724 that processes the collected electricalsignal. The first readout circuit 724 may be similar to the readoutcircuit 224 as described in reference to FIG. 2.

If the optical signal 740 is a NIR optical signal, the NIR opticalsignal propagates through the first absorption region and is received bythe second absorption region. The second absorption region receives theNIR optical signal and converts the NIR optical signal into electricalsignals. Since the thickness of the p-GeSi region 708 is generally thin(e.g., 50˜150 nm), the optical signal 740 propagates into the intrinsicGeSi region 710, where the intrinsic GeSi region 710 absorbs the opticalsignal 740 and converts the optical signal 740 into free carriers. Then+ GeSi region 712 may repel the holes generated from the intrinsic GeSiregion 710 to avoid surface recombination and thereby may increase thecarrier collection efficiency.

The photo-generated free holes in the intrinsic GeSi region 710 maydrift or diffuse into the p-Si region 706. The photo-generated freeelectrons in the intrinsic GeSi region 710 may be repelled by the p-GeSiregion 708, which prevents the free electrons from entering the p-Siregion 706. In some implementations, a drain supply voltage V_(DD) maybe coupled to the n+ GeSi region 712 to create an electric field withinthe second absorption region, such that the free holes may drift ordiffuse towards the p-Si region 706 while the free electrons maytransport to the V_(DD) voltage source.

The second gate 730 may be coupled to the second gate control signal738. For example, the second gate 730 may be coupled to a voltagesource, where the second gate control signal 738 may be a time-varyingvoltage signal from the voltage source. The second gate control signal738 controls a flow of free holes from the p-Si region 706 to the p+ Siregion 736. For example, if a voltage of the second gate control signal738 exceeds a threshold voltage, free holes accumulated in the p-Siregion 706 will drift or diffuse towards the p+ Si region 736. The p+ Siregion 736 may be coupled to the second readout circuit 732 for furtherprocessing of the collected electrical signal.

Although not shown in FIG. 7, in some other implementations, the firstabsorption region and the second absorption region may alternatively bedesigned into opposite polarity to collect holes and electrons,respectively. In this case, the p-Si substrate 702 would be replaced byan n-Si substrate, the p-Si region 706 would be replaced by an n-Siregion, the n-Si region 704 would be replaced by a p-Si region, the n+GeSi region 712 would be replaced by an p+ GeSi region, the p+ GeSiregion 708 would be replaced by a n+ GeSi region, the n+ Si region 716would be replaced by a p+ region, the n-Si region 728 would be replacedby a p-Si region, and the p+ Si region 736 would be replaced by an n+region. Although not shown in FIG. 7, in some implementations, the n+GeSi region 712 may have different polarity, namely become a p+ GeSiregion to form a p-i-p vertical doping profile in the GeSi regions.

FIG. 8 illustrates an example photodiode 800 having a hybrid structurefor detecting visible and NIR optical signals. Similar to the examplephotodiodes 700 as described in reference to FIG. 7, the photodiode 800includes a first absorption region for converting a visible opticalsignal to photo-carriers, and a second absorption region for convertinga NIR optical signal to photo-carriers. A first gate control signal 822controls the transfer of free-electrons generated by the firstabsorption region to a first readout circuit 824, and a second gatecontrol signal 838 controls the transfer of free-holes generated by thesecond absorption region to a second readout circuit 832. The photodiode800 may be one of the photodiodes in the sensor layer 108 as describedin reference to FIG. 1, for example.

In some implementations, the first absorption region may include an n-Siregion 804 and a p+ Si region 806 fabricated in a p-Si substrate 802.The second absorption region may include a p+ GeSi region 808, anintrinsic GeSi region 810, and an n+ GeSi region 812. The firstabsorption region and the second absorption region are bonded using afirst donor wafer 850 and a second donor wafer 852, and the firstabsorption region and the second absorption region are electricallycoupled by one or more interconnects 842.

If the optical signal 840 is a visible optical signal, the operations ofthe photodiode 800 is similar to the operations of the photodiode 700 asdescribed in reference to FIG. 7. If the optical signal 840 is a NIRoptical signal, the NIR optical signal propagates through the firstabsorption region, the first donor wafer 850, and the second donor wafer852, and is received by the second absorption region. The secondabsorption region receives the NIR optical signal and converts the NIRoptical signal into electrical signals. Since the thickness of the p+GeSi region 808 is generally thin (e.g., 50˜150 nm), the optical signal840 propagates into the intrinsic GeSi region 810, where the intrinsicGeSi region 810 absorbs the optical signal 840 and converts the opticalsignal 840 into free carriers. The n+ GeSi region 812 may repel theholes generated from the intrinsic GeSi region 810 to avoid surfacerecombination and thereby may increase the carrier collectionefficiency. The photo-generated free holes in the intrinsic GeSi region810 may transport to the p-Si region 806 via the one or moreinterconnects 842. The photo-generated free electrons in the intrinsicGeSi region 810 may be repelled by the p+ GeSi region 808, whichprevents the free electrons from entering the p+ Si region 806.

In some implementations, a drain supply voltage V_(DD) may be coupled tothe n+ GeSi region 812 to create an electric field within the secondabsorption region, such that the free holes may drift or diffuse towardsthe p+ Si region 806 via the interconnects 842 while the free electronsmay transport to the V_(DD) voltage source. The second gate 830 may becoupled to the second gate control signal 838. The second gate controlsignal 838 controls a flow of free holes from the p+ Si region 806 tothe p+ Si region 836. The p+ Si region 836 may be coupled to the secondreadout circuit 832 for further processing of the collected electricalsignal.

Although not shown in FIG. 8, in some other implementations, the firstabsorption region and the second absorption region may alternatively bedesigned into opposite polarity to collect holes and electrons,respectively. In this case, the p-Si substrate 802 would be replaced byan n-Si substrate, the p+ Si region 806 would be replaced by an n+ Siregion, the n-Si region 804 would be replaced by a p-Si region, the n+GeSi region 812 would be replaced by a p+ GeSi region, the p+ GeSiregion 808 would be replaced by an n+ GeSi region, the n+ Si region 816would be replaced by a p+ region, the n-Si region 828 would be replacedby a p-Si region, and the p+ Si region 836 would be replaced by an n+region.

FIG. 9 illustrates an example photodiode 900 having a hybrid structurefor detecting visible and NIR optical signals. Similar to the examplephotodiodes 700 as described in reference to FIG. 7, the photodiode 900includes a first absorption region for converting a visible opticalsignal to photo-carriers, and a second absorption region for convertinga NIR optical signal to photo-carriers. A first gate control signal 922controls the transfer of free-electrons generated by the firstabsorption region to a first readout circuit 924, and a second gatecontrol signal 938 controls the transfer of free-holes generated by thesecond absorption region to a second readout circuit 932. The photodiode900 may be one of the photodiodes in the sensor layer 108 as describedin reference to FIG. 1, for example.

In some implementations, the first absorption region may include an n-Siregion 904 and a p-Si region 906 fabricated in a p-Si substrate 902. Thesecond absorption region may include a p-GeSi region 908, an intrinsicGeSi region 910, and an n+ GeSi region 912. The p-GeSi region 908 may beformed in an etched region of an insulator layer 942 (e.g., oxide) usinga lateral strain dilution technique or an aspect ratio trappingtechnique for forming a germanium or germanium-silicon mesa havingreduced defects or being defect-free, which results into a lower darkcurrent and a better sensitivity/dynamic range. The lateral straindilution technique is described in U.S. patent application Ser. No.15/216,924 titled “High Efficiency Wide Spectrum Sensor,” which is fullyincorporated by reference herein.

If the optical signal 940 is a visible optical signal, the operations ofthe photodiode 900 is similar to the operations of the photodiode 700 asdescribed in reference to FIG. 7. If the optical signal 940 is a NIRoptical signal, the NIR optical signal propagates through the firstabsorption region, and is received by the second absorption region. Thesecond absorption region receives the NIR optical signal and convertsthe NIR optical signal into electrical signals. Since the thickness ofthe p-GeSi region 908 is generally thin (e.g., 50˜150 nm), the opticalsignal 940 propagates into the intrinsic GeSi region 910, where theintrinsic GeSi region 910 absorbs the optical signal 940 and convertsthe optical signal 940 into free carriers. The n+ GeSi region 912 mayrepel the holes generated from the intrinsic GeSi region 910 to avoidsurface recombination and thereby may increase the carrier collectionefficiency. The photo-generated free holes in the intrinsic GeSi region910 may transport to the p-Si region 906 via the p-GeSi region 908. Thephoto-generated free electrons in the intrinsic GeSi region 910 may berepelled by the p-GeSi region 908, which prevents the free electronsfrom entering the p-Si region 906.

In some implementations, a drain supply voltage V_(DD) may be coupled tothe n+ GeSi region 912 to create an electric field within the secondabsorption region, such that the free holes may drift or diffuse towardsthe p-Si region 906 via the p-GeSi region 908 while the free electronsmay transport to the V_(DD) voltage source. The second gate 930 may becoupled to the second gate control signal 938. The second gate controlsignal 938 controls a flow of free holes from the p-Si region 906 to thep+ Si region 936. The p+ Si region 936 may be coupled to the secondreadout circuit 932 for further processing of the collected electricalsignal.

Although not shown in FIG. 9, in some other implementations, the firstabsorption region and the second absorption region may alternatively bedesigned into opposite polarity to collect holes and electrons,respectively. In this case, the p-Si substrate 902 would be replaced byan n-Si substrate, the p-Si region 906 would be replaced by an n-Siregion, the n-Si region 904 would be replaced by a p-Si region, the n+GeSi region 912 would be replaced by a p+ GeSi region, the p-GeSi region908 would be replaced by an n-GeSi region, the n+ Si region 916 would bereplaced by a p+ region, the n-Si region 928 would be replaced by a p-Siregion, and the p+ Si region 936 would be replaced by an n+ region.

FIG. 10 illustrates an example photodiode 1000 having a hybrid structurefor detecting visible and NIR optical signals. Similar to the examplephotodiodes 700 as described in reference to FIG. 7, the photodiode 1000includes a first absorption region for converting a visible opticalsignal to photo-carriers, and a second absorption region for convertinga NIR optical signal to photo-carriers. A first gate control signal 1022controls the transfer of free-electrons generated by the firstabsorption region to a first readout circuit 1024, and a second gatecontrol signal 1038 controls the transfer of free-holes generated by thesecond absorption region to a second readout circuit 1032. Thephotodiode 1000 may be one of the photodiodes in the sensor layer 108 asdescribed in reference to FIG. 1, for example.

In some implementations, the first absorption region may include an n-Siregion 1004 and a p+ Si region 1006 fabricated in a p-Si substrate 1002.The second absorption region may include a p+ GeSi region 1008, anintrinsic GeSi region 1010, and an n+ GeSi region 1012. The firstabsorption region and the second absorption region are bonded using afirst donor wafer 1050 and a second donor wafer 1052, and the firstabsorption region and the second absorption region are electricallycoupled by one or more interconnects 1044. The n+ GeSi region 1012 maybe formed in an etched region of an insulator layer (e.g., oxide) 1042using a lateral strain dilution technique or an aspect ratio trappingtechnique for forming a germanium or germanium-silicon mesa havingreduced defects or being defect-free, which results into a lower darkcurrent and a better sensitivity/dynamic range. The lateral straindilution technique is described in U.S. patent application Ser. No.15/216,924 titled “High Efficiency Wide Spectrum Sensor,” which is fullyincorporated by reference herein.

If the optical signal 1040 is a visible optical signal, the operationsof the photodiode 1000 is similar to the operations of the photodiode700 as described in reference to FIG. 7. If the optical signal 1040 is aNIR optical signal, the NIR optical signal propagates through the firstabsorption region, and is received by the second absorption region. Thesecond absorption region receives the NIR optical signal and convertsthe NIR optical signal into electrical signals. Since the thickness ofthe p+ GeSi region 1008 is generally thin (e.g., 50˜150 nm), the opticalsignal 1040 propagates into the intrinsic GeSi region 1010, where theintrinsic GeSi region 1010 absorbs the optical signal 1040 and convertsthe optical signal 1040 into free carriers. The n+ GeSi region 1012 andthe insulator layer 1042 may repel the holes generated from theintrinsic GeSi region 1010 to avoid surface recombination and therebymay increase the carrier collection efficiency. The photo-generated freeholes in the intrinsic GeSi region 1010 may transport to the p+ Siregion 1006 via the one or more interconnects 1044. The photo-generatedfree electrons in the intrinsic GeSi region 1010 may be repelled by thep+ GeSi region 1008, which prevents the free electrons from entering thep+ Si region 1006.

In some implementations, a drain supply voltage V_(DD) may be coupled tothe n+ GeSi region 1012 to create an electric field within the secondabsorption region, such that the free holes may drift or diffuse towardsthe p+ Si region 1006 via the one or more interconnects 1044 while thefree electrons may transport to the V_(DD) voltage source. The secondgate 1030 may be coupled to the second gate control signal 1038. Thesecond gate control signal 1038 controls a flow of free holes from thep+ Si region 1006 to the p+ Si region 1036. The p+ Si region 1036 may becoupled to the second readout circuit 1032 for further processing of thecollected electrical signal.

Although not shown in FIG. 10, in some other implementations, the firstabsorption region and the second absorption region may alternatively bedesigned into opposite polarity to collect holes and electrons,respectively. In this case, the p-Si substrate 1002 would be replaced byan n-Si substrate, the p+ Si region 1006 would be replaced by an n+ Siregion, the n-Si region 1004 would be replaced by a p-Si region, the n+GeSi region 1012 would be replaced by a p+ GeSi region, the p+ GeSiregion 1008 would be replaced by an n+ GeSi region, the n+ Si region1016 would be replaced by a p+ region, the n-Si region 1028 would bereplaced by a p-Si region, and the p+ Si region 1036 would be replacedby an n+ region.

FIG. 11 shows a top view of an example integrated photodiode array 1100for detecting visible and NIR light as well as for a TOF application.The photodiode array 1100 includes a NIR/TOF/VIS pixel 1102. TheNIR/TOF/VIS pixel 1102 includes an NIR gate 1106, a first TOF gate 1112,a second TOF gate 1114, and a VIS gate 1108. The controls of the chargereadout using the NIR gate 1106 and the VIS gate 1108 are similar to themulti-gate photodiode 200, 300, 400, 500, 700, 800, 900, or 1000 asdescribed in reference to FIG. 2, FIG. 3, FIG. 4, FIG. 5, FIG. 7, FIG.8, FIG. 9, or FIG. 10, respectively. The controls of the charge readoutusing the TOF gates 1112 and 1114 are similar to the multi-gatephotodiode 1500 as described in reference to FIG. 15 and also aredescribed in U.S. patent application Ser. No. 15/228,282 titled“Germanium-Silicon Light Sensing Apparatus,” which is fully incorporatedby reference herein. The readout circuits coupled to the NIR gate 1106and the TOF gates 1112 and 1114 would collect the same type of carriers,and the readout circuit coupled to the VIS gate 1108 would collect theopposite type of carriers. For example, if the readout circuits of theNIR gate 1106 and the TOF gates 1112 and 1114 are configured to collectelectrons, the readout circuit coupled to the VIS gate 1108 would beconfigured to collect holes. Conversely, if the readout circuits of theNIR gate 1106 and the TOF gates 1112 and 1114 are configured to collectholes, the readout circuit coupled to the VIS gate 1108 would beconfigured to collect electrons.

FIG. 12 shows a top view of an example integrated photodiode array 1200for detecting visible light and for a TOF application. The photodiodearray 1200 includes a TOF/VIS pixel 1202. The TOF/VIS pixel 1202includes a first TOF gate 1212, a second TOF gate 1214, and a VIS gate1208. The controls of the charge readout using the VIS gate 1208 aresimilar to the multi-gate photodiode 200, 300, 400, 500, 700, 800, 900,or 1000 as described in reference to FIG. 2, FIG. 3, FIG. 4, FIG. 5,FIG. 7, FIG. 8, FIG. 9, or FIG. 10, respectively. The controls of thecharge readout using the TOF gates 1212 and 1214 are similar to themulti-gate photodiode 1500 as described in reference to FIG. 15 and alsoare described in U.S. patent application Ser. No. 15/228,282 titled“Germanium-Silicon Light Sensing Apparatus,” which is fully incorporatedby reference herein. The readout circuits coupled to the TOF gates 1212and 1214 would collect the same type of carriers, and the readoutcircuit coupled to the VIS gate 1208 would collect the opposite type ofcarriers. For example, if the readout circuits of the TOF gates 1212 and1214 are configured to collect electrons, the readout circuit coupled tothe VIS gate 1208 would be configured to collect holes. Conversely, ifthe readout circuits of the TOF gates 1212 and 1214 are configured tocollect holes, the readout circuit coupled to the VIS gate 1208 would beconfigured to collect electrons.

FIG. 13 illustrates example photodiodes 1300 for detecting visible andnear-infrared optical signals. The example photodiodes 1300 includes anNIR pixel 1350 for collecting holes and a visible pixel 1352 forcollecting electrons, where the NIR pixel 1350 and the visible pixel1352 are formed on a common substrate. The NIR pixel 1350 and thevisible pixel 1352 may not be separated by an isolation structure. TheNIR pixel 1350 is configured to detect an optical signal having awavelength in the NIR range. The visible pixel 1352 is configured todetect an optical signal having a wavelength in the visible range (e.g.,blue and/or green and/or red). The NIR pixel 1350 and the visible pixel1352 may be photodiodes in the sensor layer 108 as described inreference to FIG. 1, for example.

The visible pixel 1350 is configured to collect free electrons generatedfrom photo-generated carriers, and includes an n-Si region 1304, an n+Si region 1316, an p-Si region 1306, a first gate 1320, a first gatecontrol signal 1322 coupled to the first gate 1320, and a first readoutcircuit 1324 coupled to the n+ Si region 1316. In general, the p-Silayer 1306 receives a visible optical signal 1342. Since the thicknessof the p-Si layer 1306 is generally thin (e.g., 50˜150 nm), the opticalsignal 1342 propagates into the n-Si region 1304, where the n-Si region1304 absorbs the optical signal 1342 and converts the optical signal1342 into free carriers. In some implementations, the optical signal1342 may be filtered by a wavelength filter not shown in this figure,such as a filter in the filter layer 110 as described in reference toFIG. 1. In some implementations, a beam profile of the optical signal1342 may be shaped by a lens not shown in this figure, such as a lens inthe lens layer 112 as described in reference to FIG. 1.

In general, a built-in potential between the p-Si region 1306 and then-Si region 1304 creates an electric field between the two regions,where free electrons generated from the n-Si region 1304 aredrifted/diffused towards the region below the p-Si region 1306 by theelectric field. The first gate 1320 may be coupled to the first gatecontrol signal 1322. For example, the first gate 1320 may be coupled toa voltage source, where the first gate control signal 1322 may be atime-varying voltage signal from the voltage source. The first gatecontrol signal 1322 controls a flow of free electrons from the regionbelow the p-Si region 1306 to the n+ Si region 1316. For example, if avoltage of the first gate control signal 1322 exceeds a thresholdvoltage, free electrons accumulated in the region below the p-Si region1306 will drift or diffuse to the n+ Si region 1316 for collection. Then+ Si region 1316 may be coupled to the first readout circuit 1324 thatprocesses the collected electrical signal. The first readout circuit1324 may be similar to the readout circuit 224 as described in referenceto FIG. 2.

The NIR pixel 1350 is configured to collect free holes generated fromphoto-generated carriers, and includes an n-Si region 1328, a p+ Siregion 1336, a second gate 1330, a second gate control signal 1338coupled to the second gate 1330, a second readout circuit 1332 coupledto the p+ Si region 1336, a n+ GeSi region 1312, an intrinsic GeSiregion 1310, and a p-Ge region 1308. In addition, the NIR pixel 1350shares the p-Si region 1306 with the VIS pixel 1352, but thegermanium-silicon mesa may not be formed on the n-Si region 1304.

The n+ GeSi region 1312 receives a NIR optical signal 1340 and convertsthe NIR optical signal 1340 into electrical signals. Since the thicknessof the n+ GeSi layer 1312 is generally thin (e.g., 50˜150 nm), theoptical signal 1340 propagates into the intrinsic GeSi region 1310,where the intrinsic GeSi region 1310 absorbs the optical signal 1340 andconverts the optical signal 1340 into free carriers. In someimplementations, a thickness of the intrinsic GeSi region 1310 may bebetween 0.05 μm to 2 μm. In some implementations, the n+ GeSi region1312 may repel the holes generated from the intrinsic GeSi region 1310to avoid surface recombination and thereby may increase the carriercollection efficiency.

The photo-generated free holes in the intrinsic GeSi region 1310 maydrift or diffuse into the p-Si region 1306. The photo-generated freeelectrons in the intrinsic GeSi region 1310 may be repelled by thep-GeSi region 1308, which prevents the free electrons from entering thep-Si region 1306. In some implementations, a drain supply voltage V_(DD)may be applied to the NIR pixel 1350 to create an electric field betweenthe n+ GeSi region 1312 and the p-Si region 1308, such that the freeholes may drift or diffuse towards the p-Si region 1306 while the freeelectrons may drift or diffuse towards the n+ GeSi region 1312.

The second gate 1330 may be coupled to the second gate control signal1338. The second control signal 1338 controls a flow of free holes fromthe p-Si region 1306 to the p+ Si region 1336. For example, if a voltageof the second gate control signal 1338 exceeds a threshold voltage, freeholes accumulated in the p-Si region 1306 will drift or diffuse towardsthe p+ Si region 1336. The p+ Si region 1336 may be coupled to thesecond readout circuit 1332 for further processing of the collectedelectrical signal.

Although not shown in FIG. 13, in some other implementations, thevisible pixel 1352 may alternatively be designed into opposite polarityto collect holes instead of electrons and the NIR pixel 1350 mayalternatively be designed into opposite polarity to collect electronsinstead of holes. In this case, the p-Si substrate 1302 would bereplaced by an n-Si substrate, the p-Si region 1306 would be replaced byan n-Si region, the n-Si regions 1304 and 1328 would be replaced by p-Siregions, the p+ Si region 1336 would be replaced by an n+ Si region, then+ Si region 1316 would be replaced by a p+ Si region, the n+ GeSiregion 1312 would be replaced by a p+ GeSi region, and the p-GeSi region1308 would be replaced by an n-GeSi region. In some implementations, thedirection of light signal shown in FIG. 13 may be reversed depending ondesigns, packaging, and applications. For example, the NIR opticalsignal 1340 may enter the NIR pixel 1350 through the p-Si substrate1302, and the visible optical signal 1342 may enter the visible pixel1352 through the p-Si substrate 1302 and the n-Si region 1304.

FIG. 14 illustrates example photodiodes 1400 for detecting visible andnear-infrared optical signals. Similar to the photodiodes 1300 asdescribed in reference to FIG. 13, the example photodiodes 1400 includesan NIR pixel 1450 for collecting holes and a visible pixel 1452 forcollecting electrons, where the NIR pixel 1450 and the visible pixel1452 are formed on a common substrate. The visible pixel 1450 includesan n-Si region 1404, an n+ Si region 1416, an p-Si region 1406, a firstgate 1420, a first gate control signal 1422 coupled to the first gate1420, and a first readout circuit 1424 coupled to the n+ Si region 1416.The operations of the visible pixel 1450 is similar to the operations ofthe visible pixel 1350 as described in reference to FIG. 13.

The NIR pixel 1450 is configured to collect free holes generated fromphoto-generated carriers, and includes an n-Si region 1428, a p+ Siregion 1436, a second gate 1430, a second gate control signal 1438coupled to the second gate 1430, a second readout circuit 1432 coupledto the p+ Si region 1436, a n+ GeSi region 1412, an intrinsic GeSiregion 1410, and a p-Ge region 1408. The p-GeSi region 1408 may beformed in an etched region of an insulator layer (e.g., oxide) 1442using a lateral strain dilution technique or an aspect ratio trappingtechnique for forming a germanium or germanium-silicon mesa havingreduced defects or being defect-free, which results into a lower darkcurrent and a better sensitivity/dynamic range. The lateral straindilution technique is described in U.S. patent application Ser. No.15/216,924 titled “High Efficiency Wide Spectrum Sensor,” which is fullyincorporated by reference herein.

The n+ GeSi region 1412 receives a NIR optical signal 1440 and convertsthe NIR optical signal 1440 into electrical signals. Since the thicknessof the n+ GeSi layer 1412 is generally thin (e.g., 50˜150 nm), theoptical signal 1440 propagates into the intrinsic GeSi region 1410,where the intrinsic GeSi region 1410 absorbs the optical signal 1440 andconverts the optical signal 1440 into free carriers. In someimplementations, a thickness of the intrinsic GeSi region 1410 may bebetween 0.05 μm to 2 μm. In some implementations, the n+ GeSi region1412 may repel the holes generated from the intrinsic GeSi region 1410to avoid surface recombination and thereby may increase the carriercollection efficiency.

The photo-generated free holes in the intrinsic GeSi region 1410 maydrift or diffuse into the p-Si region 1406 via the p-GeSi region 1408.The photo-generated free electrons in the intrinsic GeSi region 1410 maybe repelled by the p-GeSi region 1408, which prevents the free electronsfrom entering the p-Si region 1406. In some implementations, a drainsupply voltage V_(DD) may be applied to the NIR pixel 1450 to create anelectric field between the n+ GeSi region 1412 and the p-Si region 1408,such that the free holes may drift or diffuse towards the p-Si region1406 while the free electrons may drift or diffuse towards the n+ GeSiregion 1412.

The second gate 1430 may be coupled to the second gate control signal1438. The second control signal 1438 controls a flow of free holes fromthe p-Si region 1406 to the p+ Si region 1436. The p+ Si region 1436 maybe coupled to the second readout circuit 1432 for further processing ofthe collected electrical signal.

Although not shown in FIG. 14, in some other implementations, thevisible pixel 1452 may alternatively be designed into opposite polarityto collect holes instead of electrons and the NIR pixel 1450 mayalternatively be designed into opposite polarity to collect electronsinstead of holes. In this case, the p-Si substrate 1402 would bereplaced by an n-Si substrate, the p-Si region 1406 would be replaced byan n-Si region, the n-Si regions 1404 and 1428 would be replaced by p-Siregions, the p+ Si region 1436 would be replaced by an n+ Si region, then+ Si region 1416 would be replaced by a p+ Si region, the n+ GeSiregion 1412 would be replaced by a p+ GeSi region, and the p-GeSi region1408 would be replaced by an n-GeSi region. In some implementations, thedirection of light signal shown in FIG. 14 may be reversed depending ondesigns, packaging, and applications. For example, the NIR opticalsignal 1440 may enter the NIR pixel 1450 through the p-Si substrate1402, and the visible optical signal 1442 may enter the visible pixel1452 through the p-Si substrate 1402 and the n-Si region 1404.

FIG. 15 is an example multi-gate photodiode 1500 for converting anoptical signal to an electrical signal. The multi-gate photodiode 1500includes an absorption layer 1506 fabricated on a substrate 1502. Thesubstrate 1502 may be any suitable substrate where semiconductor devicescan be fabricated on. For example, the substrate 1502 may be a siliconsubstrate. The coupling between the absorption layer 1506 and a first p+Si region 1512 is controlled by a first gate 1508. The coupling betweenthe absorption layer 1506 and a second p+ Si region 1514 is controlledby a second gate 1510.

In general, the absorption layer 1506 receives an optical signal 1512and converts the optical signal 1512 into electrical signals. Althoughnot shown in FIG. 15, in some implementations, the direction of theoptical signal 1512 may be reversed depending on designs, packaging, andapplications. For example, the optical signal 1512 may enter themulti-gate photodiode 1500 through the substrate 1502. The absorptionlayer 1506 is selected to have a high absorption coefficient at thedesired wavelength range. For NIR wavelengths, the absorption layer 1506may be a GeSi mesa, where the GeSi absorbs photons in the optical signal1512 and generates electron-hole pairs. The material composition ofgermanium and silicon in the GeSi mesa may be selected for specificprocesses or applications. In some implementations, the absorption layer1506 is designed to have a thickness t. For example, for 850 nmwavelength, the thickness of the GeSi mesa may be approximately 1 μm tohave a substantial quantum efficiency. In some implementations, thesurface of the absorption layer 1506 is designed to have a specificshape. For example, the GeSi mesa may be circular, square, orrectangular depending on the spatial profile of the optical signal 1512on the surface of the GeSi mesa. In some implementations, the absorptionlayer 1506 is designed to have a lateral dimension d for receiving theoptical signal 1512. For example, the GeSi mesa may have a circularshape, where d can range from 1 μm to 50 μm.

In some implementations, the absorption layer 1506 may include an n+GeSi region 1531. The n+ GeSi region 1531 may repel the holes from thesurface of the absorption region 1506 and thereby may increase thedevice bandwidth. The multi-gate photodiode 1500 includes a p-wellregion 1504 implanted in the substrate 1502.

The first gate 1508 is coupled to a first gate control signal 1522 and afirst readout circuit 1524. The second gate 1510 is coupled to a secondcontrol signal 1532 and a second readout circuit 1534. The first gate1508, the first gate control signal 1522, the first readout circuit1524, the second gate 1510, the second gate control signal 1532, and thesecond readout circuit 1534 are similar to the second gate 1428, thesecond gate control signal 1438, and the second readout circuit 1432 asdescribed in reference to FIG. 14.

The first control signal 1522 and the second control signal 1532 areused to control the collection of holes generated by the absorbedphotons. For example, when the first gate 1508 is turned “on” and thesecond gate 1510 is turned “off”, holes would drift from the p-wellregion 1504 to the p+ Si region 1512. Conversely, when the first gate1508 is turned “off” and the second gate 1510 is turned “on”, holeswould drift from the p-well region 1504 to the p+ Si region 1514. Insome implementations, a voltage may be applied between the n+ GeSiregion 1531 and the p-well 1504 to increase the electric field insidethe absorption layer 1506 for drifting the holes towards the p-wellregion 1504.

Although not shown in FIG. 15, in some other implementations, thephotodiode 1500 may alternatively be designed into opposite polarity tocollect electrons. In this case, the n-Si region 1502 would be replacedby an n-Si region, the p-well region 1504 would be replaced by an n-wellregion, the p+ Si regions 1512 and 1514 would be replaced by n+ Siregions, and the n+ GeSi region 1531 would be replaced by a p+ GeSiregion.

FIGS. 16A-16D illustrate an example design 1600 for selectively forminggermanium-silicon on a substrate. The design 1600 may be used tofabricate the photodiode array 100, 200, or 300, for example. Referringto FIG. 16A, a recess 1604 is formed on a substrate 1602. The recess1604 may define the photodiode area for an NIR pixel. The recess may beformed using lithography followed by a dry etching of the substrate1602. The shape of the recess may correspond to the shape of the pixel,such as a square, a circle, or other suitable shapes.

Referring to FIG. 16B, a dielectric layer may be deposited over thesubstrate, and a directional etch may be performed to form a sidewallspacer 1606. The directional etch may be an anisotropic dry etch.Referring to FIG. 16C, a germanium or germanium-silicon region 1608 isselectively grown from the substrate 1602. For example, thegermanium-silicon region 1608 may be formed using epitaxial growththrough chemical vapor deposition (CVD) system.

Referring to FIG. 16D, the germanium or germanium-silicon region 1608 isplanarized with the substrate 1602. The germanium or germanium-siliconregion 1608 may be planarized using chemical mechanical polishing (CMP)or any other suitable techniques. In some other implementations,although not shown in these figures, germanium may replacegermanium-silicon as the sensor material in the descriptions related toFIGS. 16A-16D.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the disclosure. For example, for theexample photodiodes described in this application, a germanium-siliconalloy may be replaced by a germanium-tin alloy for applications wherelonger operating wavelengths at the infrared region is required. Asanother example, for the example photodiodes described in thisapplication, the germanium concentration of the germanium-silicon alloymay be varied based on application or process constraints and/orrequirements. The drawings shown in this application are forillustration and working principle explanation purpose. For example,FIG. 2 does not limit that the orientation of the p+ GeSi region 212 tobe at the bottom and the p+ Si region 204 to be at the top for packagingor operation purposes. Rather, the direction of the optical signal 220would inform the orientation of the photodiode 200, i.e., the firstabsorption region would receive the optical signal 220 before the secondabsorption region.

Various implementations may have been discussed using two-dimensionalcross-sections for easy description and illustration purpose.Nevertheless, the three-dimensional variations and derivations shouldalso be included within the scope of the disclosure as long as there arecorresponding two-dimensional cross-sections in the three-dimensionalstructures.

While this specification contains many specifics, these should not beconstrued as limitations, but rather as descriptions of featuresspecific to particular embodiments. Certain features that are describedin this specification in the context of separate embodiments may also beimplemented in combination in a single embodiment. Conversely, variousfeatures that are described in the context of a single embodiment mayalso be implemented in multiple embodiments separately or in anysuitable subcombination. Moreover, although features may be describedabove as acting in certain combinations and even initially claimed assuch, one or more features from a claimed combination may in some casesbe excised from the combination, and the claimed combination may bedirected to a subcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the embodiments described above should not be understoodas requiring such separation in all embodiments, and it should beunderstood that the described program components and systems maygenerally be integrated together in a single software product orpackaged into multiple software products.

Thus, particular embodiments have been described. Other embodiments arewithin the scope of the following claims. For example, the actionsrecited in the claims may be performed in a different order and stillachieve desirable results.

What is claimed is:
 1. An optical sensor comprising: a semiconductorsubstrate comprising a recess; a first light absorption region formed inthe semiconductor substrate, the first light absorption regionconfigured to absorb photons at a first wavelength range and to generatephoto-carriers from the absorbed photons; a second light absorptionregion comprising an intrinsic region, the intrinsic region being formedon the first light absorption region, and the second light absorptionregion configured to absorb photons at a second wavelength range and togenerate photo-carriers from the absorbed photons, wherein the secondlight absorption region includes a mesa including germanium; and asensor control signal coupled to the second light absorption region, thesensor control signal configured to provide at least a first controllevel and a second control level, wherein at the first control level, anenergy band difference at an interface of the first light absorptionregion and the second light absorption region is above a threshold forblocking one specific polarity of the photo-carriers generated by thesecond light absorption region from entering the first light absorptionregion, and wherein at the second control level, the energy banddifference at the interface of the first light absorption region and thesecond light absorption region is below the threshold for blocking theone specific polarity of the photo-carriers generated by the secondlight absorption region from entering the first light absorption region,and wherein at least a portion of the second light absorption region isembedded in the recess.
 2. The optical sensor of claim 1, wherein thefirst light absorption region comprises: an n-doped silicon region; anda p-doped silicon region on the n-doped silicon region, and wherein theintrinsic region of the second light absorption region includesgermanium and is formed on the p-doped silicon region of the first lightabsorption region, and the second light absorption region comprises ap-doped region including germanium on the intrinsic region.
 3. Theoptical sensor of claim 2, further comprising: an n-doped readout regioncoupled to a readout circuit; and a gate coupled to a gate controlsignal, the gate configured to control a carrier transit between thefirst light absorption region and the n-doped readout region.
 4. Theoptical sensor of claim 1, wherein a surface of the second lightabsorption region is flush with a surface of the semiconductorsubstrate.
 5. The optical sensor of claim 1, wherein the recess furthercomprises a sidewall spacer.
 6. An optical sensor comprising: asubstrate comprising a recess; a first diode formed using a firstmaterial, the first diode comprising an n-doped region and a p-dopedregion; a NMOS transistor comprising: a source region coupled to then-doped region of the first diode; a gate region coupled to a NMOS gatecontrol signal; and a drain region; a second diode formed using a secondmaterial different from the first material, wherein a surface of thesecond diode is flush with a surface of the substrate, the second diodecomprising: an n-doped region coupled to a first bias signal; and ap-doped region; and a PMOS transistor comprising: a source regioncoupled to the p-doped region of the first diode and the p-doped regionof the second diode; a gate region coupled to a PMOS gate controlsignal; and a drain region, wherein the drain region of the NMOStransistor is coupled to a first readout circuit, and the drain regionof the PMOS transistor is coupled to a second readout circuit differentfrom the first readout circuit, wherein the first diode, the NMOStransistor, and the PMOS transistor are formed in the substrate, andwherein at least a portion of the second diode is embedded in therecess.
 7. The optical sensor of claim 6, wherein the drain region ofthe PMOS transistor is coupled to a second bias source, such that (i)the first readout circuit collects, stores, and processes electronsgenerated by the first diode, (ii) the drain region of the PMOStransistor transfers holes generated by the first diode to the secondbias source, and (iii) the drain region of the PMOS transistor transfersholes generated by the second diode to the second bias source.
 8. Theoptical sensor of claim 7, wherein the first diode is configured toabsorb light at visible wavelengths to generate electrons and holes. 9.The optical sensor of claim 6, wherein the drain region of the NMOStransistor is coupled to a third bias source, such that (i) the drainregion of the NMOS transistor transfers electrons generated by the firstdiode to the third bias source, (ii) the second readout circuitcollects, stores, and processes holes generated by the first diode, and(iii) the second readout circuit collects, stores, and processes holesgenerated by the second diode.
 10. The optical sensor of claim 9,wherein the second diode is configured to absorb light at near-infraredor infrared wavelengths to generate electrons and holes.
 11. The opticalsensor of claim 6, wherein the first diode is a diode including siliconand the second diode is a diode including germanium.
 12. The opticalsensor of claim 6, wherein the recess further comprises a sidewallspacer.
 13. An optical sensor comprising: a semiconductor substratecomprising a recess; a first light absorption region formed in thesemiconductor substrate, the first light absorption region configured toabsorb photons at a first wavelength range and to generatephoto-carriers from the absorbed photons, the first light absorptionregion comprising: a first carrier-collection region configured tocollect electrons; and a second carrier-collection region configured tocollect holes; a second light absorption region on a portion of thefirst light absorption region, the second light absorption regionconfigured to absorb photons at a second wavelength range and togenerate photo-carriers from the absorbed photons; a first readoutregion coupled to a first readout circuitry, the first readout regionconfigured to provide the electrons collected by the firstcarrier-collection region to the first readout circuitry, wherein theelectrons collected by the first carrier-collection region are providedby the first light absorption region; a first gate coupled to a firstcontrol signal that controls a carrier transport between the firstcarrier-collection region and the first readout region; a second readoutregion coupled to a second readout circuitry, the second readout regionconfigured to provide the holes collected by the secondcarrier-collection region to the second readout circuitry, wherein theholes collected by the second carrier-collection region are provided bythe second light absorption region; and a second gate coupled to asecond control signal that controls a carrier transport between thesecond carrier-collection region and the second readout region, whereinat least a portion of the second light absorption region is embedded inthe recess.
 14. The optical sensor of claim 13, wherein the second lightabsorption region comprises: a p-doped region including germanium on thefirst light absorption region; an intrinsic region including germaniumon the p-doped region; and an n-doped region including germanium on theintrinsic region.
 15. The optical sensor of claim 14, wherein thep-doped region including germanium has a first strain and a first area,and wherein the intrinsic region including germanium has a second strainthat is lower than the first strain, and the intrinsic region includinggermanium has a second area that is larger than the first area.
 16. Theoptical sensor of claim 14, wherein the second light absorption regionis on a portion of the second carrier-collection region but not on thefirst carrier-collection region, wherein the first light absorptionregion and the second light absorption region are configured to receivelight at different locations.
 17. The optical sensor of claim 16,wherein the p-doped region including germanium has a first strain and afirst area, and wherein the intrinsic region including germanium has asecond strain that is lower than the first strain, and the intrinsicregion including germanium has a second area that is larger than thefirst area.
 18. The optical sensor of claim 13, further comprising: athird readout region coupled to a third readout circuitry, the thirdreadout region configured to provide the holes collected by the secondcarrier-collection region to the third readout circuitry; and a thirdgate coupled to a third control signal that controls a carrier transportbetween the second carrier-collection region and the third readoutregion.
 19. The optical sensor of claim 18, further comprising: a fourthreadout region coupled to a fourth readout circuitry, the fourth readoutregion configured to provide the holes collected by the secondcarrier-collection region to the fourth readout circuitry; and a fourthgate coupled to a fourth control signal that controls a carriertransport between the second carrier-collection region and the fourthreadout region.
 20. The optical sensor of claim 13, wherein a surface ofthe second light absorption region is flush with a surface of thesemiconductor substrate.
 21. The optical sensor of claim 13, wherein therecess further comprises a sidewall spacer.
 22. An optical sensorcomprising: a substrate comprising a recess; a first diode formed usinga first material, the first diode comprising an n-doped region and ap-doped region; a NMOS transistor comprising: a source region coupled tothe n-doped region of the first diode; a gate region coupled to a NMOSgate control signal; and a drain region; a second diode formed using asecond material different from the first material, the second diodecomprising: an n-doped region coupled to a first bias signal; and ap-doped region, wherein the second diode further comprises an intrinsicregion, wherein the p-doped region of the second diode is on the p-dopedregion of the first diode, wherein the intrinsic region of the seconddiode is on the p-doped region of the second diode, and wherein then-doped region of the second diode is on the intrinsic region of thesecond diode; and a PMOS transistor comprising: a source region coupledto the p-doped region of the first diode and the p-doped region of thesecond diode; a gate region coupled to a PMOS gate control signal; and adrain region, wherein the drain region of the NMOS transistor is coupledto a first readout circuit, and the drain region of the PMOS transistoris coupled to a second readout circuit different from the first readoutcircuit, wherein the first diode, the NMOS transistor, and the PMOStransistor are formed in the substrate, and wherein at least a portionof the second diode is embedded in the recess.